The contrast ratio of a liquid crystal display, directly related with its adaptability, significantly affects how it would be accepted by the market. The contrast ratio is a ratio of the luminance of the brightest color (white) to that of the darkest color (black) of the display. Generally, the insufficient dark state is a main factor limiting the contrast ratio of the liquid crystal display. With the increase of a viewing angle of a thin film transistor-liquid crystal display (TFT-LCD), the contrast ratio of pictures is continually reduced, and the sharpness of the pictures also correspondingly declines. This is due to the fact that the birefringence of liquid crystal molecules in a liquid crystal layer is changed along with the viewing angle. With a compensation achieved by adopting a wide-view compensation film, light leakage of dark-state pictures can be effectively reduced, and the contrast ratio of the pictures can be greatly improved within a certain viewing angle. Generally, the compensation film functions based on the principle that it offsets the phase difference generated by a liquid crystal under different viewing angles, so as to symmetrically compensate the birefringence performance of the liquid crystal molecules.
The compensation film adopted should be differentiated regarding different liquid crystal display modes, and the compensation film used in a large-sized liquid crystal television mostly aims at a vertical alignment (VA) display mode.
As the compensation value of the compensation film varies, the status of dark-state light leakage under a large viewing angle also varies, and thus the contrast ratio differs within the same length of optical path difference (LCΔNd) of a liquid crystal.
For example, FIG. 1 shows a corresponding diagram of dark-state light leakage distribution in the prior art when the optical path difference in liquid crystal (LCΔNd) is 333.5 nm, and FIG. 2 shows a diagram of full-view contrast ratio distribution. In FIG. 1 and FIG. 2, the optical path differences in liquid crystal, the pre-tilt angles of the liquid crystal and the compensation values of an A-plate and a C-plate are shown in table 1.
TABLE 1compensationcompensationin-planevalue Rthvalue Rthopticalcompensationfor optical pathfor optical pathpathpre-tiltvalue Rodifference indifference indifferenceangle offor opticalthe thick-the thick-in liquidliquidpath differenceness directionness directioncrystalcrystalof A-plateof A-plateof C-plate333.5 nm89 degrees109 nm55 nm403 nm
Thus it could be seen that when A-plate and C-plate compensation values in the prior art are adopted, a serious light leakage phenomenon will be observed when viewing is taken in a dark state under a large angle. Therefore, the contrast ratio is lowered, and the range of the viewing angle is reduced. As a result, the sharpness of images would be greatly affected under some viewing angles.